There are two sorts of golf balls. One is a solid golf ball such as two-piece solid golf ball, or a three-piece solid golf ball, etc. and is composed of a core made of a integrally molded rubber member and a thermoplastic resin cover made from ionomer resin which cover the core. The other is a thread wound golf ball and is produced by winding a thread rubber around a solid or liquid center and, followed by covering the center with a cover having a thickness of 1 to 2 mm of ionomer resin, balata, etc. Among the solid golf balls, the two-piece solid golf ball comprising a core and a cover are popular in the market because theses balls can be easily produced. The two-piece solid golf ball obtains long flight distance because of high ball velocity at the time of hitting, and is superior in durability and flight performance to the thread wound golf ball. Accordingly, the two-piece solid golf ball is used by many golfers, exclusively amateur golfers. On the contrary, the two-piece solid golf ball has poor shot feel and lacks controllability for approach shots because it produces a very small amount of spin amount. Therefore, the two-piece solid golf ball is not accepted by the professional golfers and high-level amateur golfers, who think that shot feel at the time of hitting and controllability are important.
For solving the defects of the two-piece solid golf ball, it is suggested to make the solid core in a two layer structure to form a three-piece solid golf ball. The solid core having the two-layer structure is described in Japanese Patent Kokai Nos. 241464/1985, 181069/1987 and 80377/1989, and the structural feature used in the Japanese Patents is that the hardness of the outer layer core is set to a value higher than that of the inner layer core. That is, the amount of deformation of the golf ball is increased by making the hardness of the outside of the core higher and gradually decreasing the hardness from the outside to the inside, thereby obtaining soft shot feel. In this structure, however, the durability of the golf ball is not satisfactory.
Japanese Patent Kokal No. 23069/1994 suggests a similar three-piece structure wherein the hardness of the outer layer core is set at a value lower than that of the inner layer core. In this structure, with respect to hardness distribution, the hardness of the outside is highest and the hardness decreases gradually from the outside to inside. Therefore, there remains a problem that the rebound performance of the inner layer core is poor and the flight distance is short.